1. Field of the Invention
The invention relates to the field of photonics and in particular to tapered dielectric slab waveguides for input and output coupling of light into photonic crystal devices.
2. Description of the Prior Art
Photonic crystals are very advantageous for use in small optical devices. Recently, it has become possible to microfabricate high reflectivity mirrors by creating two- and three-dimensional periodic structures. These periodic “photonic crystals” can be designed to open up frequency bands within which the propagation of electromagnetic waves is forbidden irrespective of the propagation direction in space and to define photonic bandgaps. When combined with high index contrast slabs in which light can be efficiently guided, microfabricated two-dimensional photonic bandgap mirrors provide the geometries needed to confine light into extremely small volumes.
For example, two dimensional Fabry-Perot resonators with microfabricated mirrors are formed when defects are introduced into the photonic bandgap structure. It is then possible to tune these cavities lithographically by changing the precise geometry of the microstructures surrounding the defects. By using the same microfabrication techniques, it is also possible to guide, bend, filter and sort light in two dimensional photonic crystals. For example, by introducing line defects, photonic crystal waveguides can be constructed, and light can be guided around sharp corners without the normally associated bend losses.
Although photonic crystal devices show promise for future optical devices, the coupling of light into and out of the optical devices made from photonic crystals has been inefficient. This is due to the very small size of these devices (typically 0.2 μm) compared to the size of the input beam (for example, from an optical fiber) and the size of the output device (for example, also an optical fiber). While the typical thickness of a photonic crystal waveguide is about 0.2 μm, the smallest size of the light beam normally coupled into such a photonic crystal waveguides is around 1.0 μm. This results in a large insertion loss.
What is needed is some means whereby such insertion losses can be avoided or minimized.